Gentics & Molecular Biology exam 2

Pleiotropy

The expression of one gene has multiple phenotypic effects.

Sex limited inheritance

• autosomal genes

• Homozygous genotypes exhibit one type in males and another in females

• expression limited to one sex

Sex influenced inheritance

Same as sex limited but not actually limited, sex influences the expression of the phenotype.

Inheritance influence: Environment

• changes phenotypic expression

• assumes genotype is directly expressed but organisms develop under external environmental influences.

Penetrance

Degree of expression, % of individuals that show some degree of expression.

Expressivity

the degree to which trait expression differs among individuals. (mutants)

Genetic background

all genes in genome, “background genes”.

Position Effect

The physical location of the gene in relation to other genetic material once moved.

If genes linked on same chromosome segregate together, there are 3 meiotic cosequences:

A. Independent assortment

B. Linkage without crossing over

C. Linkage with crossing over

A. Independent assortment

• no linkage exhibited

• form 4 genetically different gametes in equal proportions, each with a different combination of alleles

• Alleles independently sort from each other

B. Complete linkage

• 2 genes on a single pair of homologs with no exchange

• only 2 different kinds of gametes are formed

• each gamete receives the alleles present on one homolog or the other, which is transmitted intact as a result of the segregation

• no crossing over between genes

• produces parental gametes

C. Crossing over

• 2 genes on a single pair of homologs with an exchange

• crossover involves only 2 non-sister chromatids of the 4 chromatids present in the tetrad

• both parental and recombinant (crossover) gametes are produced

• 2 new allele combos

Things to know about recombination:

• Almost always get a % of offspring resulting from recombination

• the frequency with crossing over occurs between any 2 linked genes is proportional to the distance separating the respective loci on chromosome

• As the distance between 2 genes increases, the proportion of recombinant gametes increases and parental gametes decrease

• This happens because the further the distance, more space for crossover

Chance of crossover increases when…

Distance is greater

Who invented chromosome mapping?

Morgan and Sturtevant

What is the source of separation discovered in Morgan’s experiments?

• Synapsed homologous chromosomes during meiosis can become wrapped around each other creating chiasmata.

• He reasoned that chiasmata are points of genetic exchange.

• this led to recombinant phenotypes.

• linked genes are arranged in linear sequences along the chromosome.

Why did the frequency vary in Morgan’s experiments?

• Morgan reasoned that 2 genes close to each other are less likely to have chiasmata form

• % of recombinant gametes is based on distance

• 3 genes needed for mapping

• estimated distance in map units (mu) & centimorgans (cM).

• mu = 1% recombination between genes on chromosome.

Single crossover (SCO)

• Distance matters!

• The closer the genes are, the less likely a single crossover event happens (a).

• Genes further apart yields gametes with traits that are recombined (b).

• When single crossover event occurs between sister chromatids, the other of the tetrad are not involved.

• The theoretical limit of crossing over is 50%

• 2 noncrossover parental gametes and 2 recombinant gametes are produced.

the % of tetrads involved in an exchange between 2 gametes is ______

twice as great.

(EX: 20% recombinant = 40% of tetrads)

Ideas from multiple crossovers:

• Have extensive chromosome maps that can determine sequence and distance.

• It is possible in a single tetrad to have multiple exchanges between non-sister chromatids.

• Double exchanges result from: Double crossovers (DCOs)

• To study DCOs: must have 3 gene pairs, heterozygous for 2 alleles

Product Law of Probablitity

the probability of two independent events occurring together can be calculated by multiplying the individual probabilities of each event occurring alone.

Likeliness of crossover events happening:

NCO > SCO > DCO

Degree of inaccuracy _______ as distance increases

Increases

Phenomenon of Interference

• inhibition of further crossover events in a nearby region of the chromosome.

• need to calculate coincidence ( c ).

• c = observed DCOs / Expected DCOs

• to quantify I: I = 1-c

• If I positive, than fewer DCO events occurred than expected.

• If I negative, more DCO events occurred than expected.

Interference ______ as genes are located further apart

Decreases

Chromosome mutations (Aberration)

• alteration in the genome resulting from a deletion, duplication, or rearrangement of chromosomal contents of an organism.

Three types of chromosome alterations:

• Aneuploidy - the gain or loss of 1 or more chromosomes

• Monosomy - loss of 1 chromosome

• Trisomy - gain 1 chromosome

Loss/addition of chromosomes causes:

• Originates as a random error during the production of gametes (nondisjunction).

• Random homologs fail to disjoin

• nondisjunction during meiosis I or II

loss (45, X)

Turner’s syndrome

Haploinsufficiency

Requirement for 2 wild type copies of a gene

addition (47, 21 +)

Down syndrome

DSCR: Down Syndrome Critical Region

Chromosome 21

Amniocentesis or chronic villus sampling (cVs)

fetal cells & DNA are obtained from maternal circulations

( has a noninvasive form: NIPGD)

Euploidy

One or more haploid sets

Polyploidy

More than 2 haploid sets

Autopolyploidy

• addition of one or more extra sets of chromosomes

• Identical to normal haploid complement of the same species

• Cause: failure of all chromosomes to segregate during meiotic divisions can produce a diploid gamete. If fertilized, zygote has 3 sets of chromosomes

• Can produce autoteraploids

Allopolyploidy

• The combination of chromosomes from different species occurs as a consequence of hybridization.

Allotetraploid

• polyploid that contains 4 haploid genomes derived from separate species.

Endopolyploidy

• a condition where only certain cells in a diploid organism are polyploid

• set of chromosomes replicate repeatedly without nuclear division

Deletions & Duplications

The total amount of genetic information in chromosome changes.

Inversions & Translocations

Genetic material remains the same but rearranged.

Deletion (deficiency)

• Chromosome breaks and pieces remain missing

• If occurs near end: Terminal

• If occurs interior: Intercalary deletion

• For synapsis to occur with large deletion homolog buckles into a compensation loop

Duplication

• Repeated segments of chromosome in the genome

• single locus present more than once

• Arises from uneven crossing over or replication error

• Results in gene redundancy

Copy number variants (CNVs)

sequences of the genome are repeated

Inversions

• rearrangement of linear gene sequence

• no loss of genetic information

• segment of chromosome turned 180 degrees within chromosome

• requires 2 breaks in chromosome and reinsertion inverted segment

Translocations

Movement of chromosomal segment to new location in genome

Reciprocal translocations

Involves exchange of segments between 2 non homologous chromosomes

Nonreciprocal translocations

Involves segments moved from one chromosome to the other.

Robertsonian Translocation

• Involves breaks at extreme ends of the short arms of 2 non homologous chromosomes

• small segments are lost

• large metacentric/submetacentric chromosome produced

Fragile-X syndrome (Martin-Bell syndrome)

caused by trinucleotide repeats in FMR1 gene

Organelle heredity

inheritance arsing from chloroplast and mitochondria.

Heteroplasmy

Variation in the content of organelles

DNA molecule must be able to:

• Replicate

• Store info

• Express information

• Variations

Central dogma of molecular biology

DNA → RNA → Proteins

Purines

Guanine & Adenine

Pyrimidines

Cytosine, Thymine & Uracil

DNA needs

1. a nitrogenous base

2. a pentose sugar

3. a phosphate group

Phosphodiester bond

phosphate group linked to 2 sugars

Oligonucleotides

short chains

Polynucleotides

Large chains

Watson + Crick

• proposed structure of DNA

• Was mostly right just stole from Rosalind

Chargaff

proposed composition of base of DNA

Rosalind Franklin

Used X-ray diffraction to show characteristic of double helix

Semiconservative replication

• storage of genetic info in a sequence of bases

• mutations or genetic changes that could result in the alternation of bases

• Proposed by Watson & Crick